1. Field of the Invention
The present invention relates to a focus detection device for detecting focus condition of an objective lens with respect to an object by sensing light from the object which has passed through the objective lens of a camera.
2. Prior Art
A focus detection device has already been proposed in which object light rays having passed through first and second areas of an objective lens that are symmetric with each other with respect to the optical axis of an objective lens, are re-imaged to form two images after once having been focused to form an image of the object, and relative positions of the two images are detected to determine how much and in which direction the focused position of the object image deviates from a predetermined focal plane (whether the focused position is in front or in the rear of the predetermined focal plane, that is, whether the focus condition is a front focus or a rear focus). A typical optical system of such a focus detection device has such a construction as shown in FIG. 13. The optical system includes a condenser lens 6 on a predetermined focal plane 4 located behind an objective lens 2 or in a position behind the focal plane, and further includes re-imaging lenses 8 and 10 behind the condenser lens 6. On focal planes of the re-imaging lenses 8 and 10 are disposed image sensors 12 and 14 which have, for example, CCDs as light detectors. The images on the image sensors 12 and 14 are nearer or closer to an optical axis 1 and to each other as shown in FIG. 14 when object images 9 and 11 are formed in front of the predetermined focal plane, that is, in the case of front focus, while in the case of rear focus those images are distant from the optical axis 1. When the objective lens 2 is in just focus or in-focus condition, the distance between two corresponding points of the two images 9 and 11 has a specific value determined by the construction of the optical system of the focus detection device. Basically, therefore, a focus condition can be determined by detecting the distance between the two corresonding points of the two images.
In an automatic focusing device of a camera which incorporates a focus detecting optical system of the type mentioned above, the sequence of integrating object light quantities by CCD image sensors, detecting and calculating a focus condition (calculating the amount of defocus) using outputs of the CCD image sensors, driving lens according to the amount of defocus and stopping it at in-focus position (shutter release in the case of the shutter button being depressed), is controlled in accordance with a program by means of a control circuit which is constituted by a microcomputer.
The above automatic focusing device performs the aforementioned sequential automatic focusing (AF) control even when the object image is about to be in focus so that in-focus position can be set finally exactly.
In effecting focus detection, the spacing between the images on the image sensors 12 and 14 must be detected. To this end, correlation is determined with respect to two light intensity destributions on the image sensors 12 and 14. Outputs of the light detectors which constitute the image sensors 12 and 14 are shifted relative to one another to obtain an amount of shift which affords the best correlation (i.e. spacing of the two images).
A considerable time is required for the calculation of such correlation because this calculation is performed by relatively shifting the outputs over a wide range. As a result, there arise problems such as deteriorated response characteristic of the automatic focusing device and deteriorated follow-up performance for a moving object.
The follow-up performance for a moving object will now be considered. In such automatic focusing device as mentioned above, if an amount of defocus is detected by a single focus detection and the objective lens is moved to in-focus position on the basis of the said amount of defocus, for example, when an object is approaching or going away from the camera, it will be impossible to obtain a just focus or in-focus condition because of movement of the object during that period.
This is as illustrated in FIG. 15, in which time base is plotted along the axis of abscissa and the amount of defocus plotted along the axis of ordinate. In this figure, a curve l shows how the amount of defocus on the film surface increases as the object approaches at a constant speed, and a straight line m indicates results obtained by following up positions in which the objective lens was about to focus. I in the time axis represents a period of time for the light integration (charge accumulation) by the image sensors and C represents a period of time for the calculation of the amount of defocus. The object approaches the camera even during the calculation time period, so even if the lens is driven on the basis of the result of the calculation, the object has already moved at the time the lens has stopped. With approach of the object, the amount of defocus increases and soon goes out of the depth of field, resulting in out-of-focus.
It is therefore necessary to make the calculation time period as short as possible.
In Japanese Patent Laid Open No. 126517/84 there is proposed a method in which a detection zone of a standard area of an image sensor is divided into three blocks and outputs of those blocks are shifted relative to the whole region of output of a reference area of the image sensor to determine an amount of shift which affords best correlation for each of those blocks and to find one corresponding to the highest correlation as a whole from the amounts of shift thus determined for those blocks. According to such proposed method, however, it is impossible to attain an effective shortening of time because at every detection of focus the output of each block of the standard area is subjected to calculation over the whole range of the output of the reference area.
In Japanese Patent Laid Open No. 75607/81 there is proposed a method in which the maximum amount of shift is determined using the amount of shift found as affording the highest correlation at the first time focus detection because the amounts of shift to be found as affording the highest correlation at the second time and the following focus detections are not larger than the amount of shift at the first time focus detection. In the case of a large amount of shift, however, it is necessary that the calculation be performed over a wide range, thus resulting in much time required for the calculation; besides, a more complicated control results because the amount of shift must be changed each time. Further, since it is impossible to cope with infeasible condition of focus detection, there is fear that it would become no longer possible to effect a continuous automatic focusing.